RE: [RFC] dmaengine/dma_slave: add context parameter to prep_slave_sg callback

On Mon, 2012-01-30 at 08:55 -0800, Bounine, Alexandre wrote:

On Monday, January 30, 2012 at 4:31 AM, Vinod Koul wrote:

quoted

On Thu, 2012-01-26 at 16:22 -0500, Alexandre Bounine wrote:

quoted

As we agreed during our discussion about adding DMA Engine support for RapidIO
subsystem, RapidIO and similar clients may benefit from adding an extra context
parameter to device_prep_slave_sg() callback.
See https://lkml.org/lkml/2011/10/24/275 for more details.

Adding the context parameter will allow to pass client/target specific
information associated with an individual data transfer request.

In the case of RapidIO support this additional information consists of target
destination ID and its buffer address (which is not mapped into the local CPU
memory space). Because a single RapidIO-capable DMA channel may queue data
transfer requests to different target devices, the per-request configuration
is required.

The proposed change eliminates need for new subsystem-specific API.
Existing DMA_SLAVE clients will ignore the new parameter.

This RFC only demonstrates the API change and does not include corresponding
changes to existing DMA_SLAVE clients. Complete set of patches will be provided
after (if) this API change is accepted.

This looks good to me. But was thinking if we need to add this new
parameter for other slave calls (circular, interleaved, memcpy...)

I agree that cyclic and interleaved calls may benefit from adding that parameter as well.
Benefits to the cyclic call are straightforward - same as dma_slave.
Adding a context parameter to the interleaved transfers may be more future proofing option
than an immediate need. Memcopy and other calls that deal with local memory transfers
probably should be left untouched.

What if we limit modifications to:
1) three calls (slave, cyclic and interleaved) OR
2) two (slave and cyclic) at this moment?

I am just more focused on dma_slave just because it fits well to provide RDMA
over RapidIO fabric.

If everybody agrees, I can go ahead and make changes to all three at once.

For now we need at least slave and cyclic, so pls go ahead and make these changes.
For interleaved, we might need it sooner [1], but I would think it would
need few more changes to the API, so it can be rolled as part of those
changes.

-- 
~Vinod

[1]: https://lkml.org/lkml/2012/1/30/48